The applicant has successfully applied an in vivo fluorescence microscopy technique to the study of permeability of microvessels in the rat small intestine and stomach. The small intestine or stomach of an anesthetized rat is exteriorized and epilluminated via a microscope with a vertical illuminator and appropriate filters. Fluorescein isothiocyanate conjugated to serum albumin is injected i.v. The fluorescent image of the microvessels is visualized on a videomonitor by using an extremely light sensitive SIT videocamera. When an agent affecting permeability, such as histamine, is applied topically to the area under study, leak of the conjugate from microvessels is seen. The extent of leakage is quantitated by determining the area and number of leaks. This technique will be used to: I - Determine the effect of known mediators of inflammation - bradykinin, serotonin, prostaglandin, leukotrienes and histamine - on rat small intestine microvascular permeability and to determine the effect of vasoconstriction and vasodilatation on a permeability increase produced by another agent. II - Determine which mediators are responsible for the increased permeability seen in gastric mucosal injury by topical alcohol. III - Determine whether any of the gut neuropeptides affect microvascular permeability or are vasodilators or vasoconstrictors in the rat small intestine. For those peptides increasing permeability, determine if this permeability effect is mediated by histamine or serotonin. Finally, determine if there is a possible physiologic role for gut neuropeptides in vascular responses such as a) escape from adrenergic vasoconstriction, b) the prompt gastric submucosal arteriolar dilatation produced by vagal stimulation, and c) the increase in gastric mucosal blood flow secondary to acid secretion.